390 Allen and Crenshaw — Sulphides of Zinc, 



for the colors from red to blue-green and negative from blue- 

 green to blue, the wave-length for which it is isotropic being 

 about 523 fi/x. Above 500 pp (toward the violet), absorption is 

 so strong that the double refraction has not been measured. 

 By transmitted light, therefore, clear crystals are pure yellow. 

 Plates O02""" thick are opaque for all the blue and the violet 

 light that cau be obtained from a 20-ampere arc through a 

 monochromatic illuminator. The ordinary ray is more strongly 

 absorbed in the green than the extraordinary ray, for which 

 reason the mineral is pleochroie. In white light the pleo- 

 chroism is not perceptible in single crystals, but in thin twin 

 crystals it may be seeu by contrast in the two parts. 



A closer study of the light-absorption of greenockite shows 

 that below (toward the red) wave-length 519 ftfi there is very 

 little absorption for ca, and from 517 to 511 it increases to 

 nearly complete opacity. For e absorption begins near 512 

 and increases similarly to 506. The change of optical sign is 

 the natural accompaniment of the more rapid increase in 

 refractive index of a> than of e near this region of absorption. 



The refractive indices for several different wave-lengths in 

 the red and orange were measured under the microscope. 

 The results are plotted on the diagram. The values for 

 sodium and lithium lights with a probable error of ± *003 are 

 as follows: e Na =2-529 ; o> Na =2-506 ; e Li =2456 ; o> Li =2-431. 

 The values used for the curves in the yellow and green are 

 extrapolated on the basis of the absorption, and of the follow- 

 ing values for double refraction obtained by measurements on 

 prisms - 03 to •04 mm thick, in monochromatic light. 



671 (Li) = -025 527 = "006 



589 (Na) = -023 523 = -000 



547 ='018 518 = — -006 



535 (Tl) = -013 516 = — -016 



Color of Cadmium sulphide. 



The color of a substance as seen by reflected light depends 

 upon the character and relative amounts of light reflected 

 directly from external surfaces and indirectly from internal 

 surfaces, that is, after having passed into the substance and 

 reflected out. The quality of the light reflected in each case 

 depends upon the refractive and absorptive powers of the sub- 

 stance, and upon the character (plainness, size, brightness) of 

 the reflecting surfaces. 



Greenockite. — This mineral absorbs all the blue and violet 

 and part of the green of the spectrum, and freely transmits 

 the rest. "When the greenockite grains seen in mass have 

 diameters of - 2 to TO or more and are bounded by plane, 

 bright faces, a comparatively large amount of blue light is 



